With the exponential growth of wireless communication, new techniques are needed to handle the high capacity of voice and data carried over wireless communication networks. The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) (referred to as “LTE” hereinafter) is a promising network proposal to meet the challenge of increased traffic.
For LTE, the orthogonal frequency-division multiplexing (OFDM) modulation scheme is chosen for the transmission of the downlink signals between a transmitter, such as a base station, and a terminal/receiver, such as a user equipment (UE) (e.g., mobile communication devices such as cell phones, etc.). Meanwhile, a special type of modulation method, which is termed single-carrier frequency-division multiple access (SC-FDMA), is used for the transmission of uplink signals.
LTE can be operated in both frequency-division duplex (FDD) and time-division duplex (TDD) modes. In FDD mode, the uplink and downlink signals are transmitted simultaneously, but in separate frequency bands. In TDD mode, the uplink and downlink signals are transmitted in the same frequency band, but in different time slots. Compared to FDD, TDD has the advantage that the downlink-to-uplink ratio can be dynamically adjusted according to the actual amounts of uplink and downlink traffic, and hence enables a more efficient use of the spectrum, especially under asymmetric operations, i.e. when the amounts of uplink and downlink traffic are different. Another advantage is that the uplink and downlink radio paths are likely to be substantially similar in the case of a slow fading system, and it means that techniques such as beamforming work well with TDD systems.
Detailed information on LTE, TDD and FDD can be found in Rumney, LTE and the Evolution of 4G Wireless, John Wiley, © 2009, and Sesia, LTE: The UMTS Long Term Evolution, Wiley © 2009, and the standard documents for E-UTRA: 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation;” 3GPP TS 36.212: “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding;” 3GPP TS 36.213: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures” the disclosures of which are incorporated by reference herein.
In most wireless communication systems, the baseband signal at the receiver needs to be converted from an analog format into a digital format so that useful information can be conveniently recovered via a sequence of digital processes. The common device that achieves this conversion is an analog-to-digital converter (ADC). Given the number of output bits of the ADC, if the power of the input signal is too large, the output of the ADC may be saturated or ‘clipped’. On the other hand, if the power of the input signal is too small, the input signal may suffer from a significant degradation in the signal-to-noise ratio (SNR) due to the quantization process of the ADC. In both cases, the information to be recovered at the received end may be lost, and a common approach to solve this problem is to apply a dynamically adjustable gain amplifier responsive to the ADC so that the magnitude of the input signal to the ADC is maintained at a desired level. This process of maintaining the input signal magnitude is a closed-loop mechanism, and is known as automatic gain control (AGC).
Due to various reasons, such as variations in the number of active users in a time slot, the mean path loss between the transmitter and receiver, constructive and destructive interferences between multiple transmission paths between the transmitter and receiver (so-called fast-fading) and variations in the data rate, there is a potentially large slot-to-slot variation in the received power. Owing to this power variation, the AGC is required to be able to set the gain of the amplifier preceding the ADC quickly and correctly. Otherwise, the data at the beginning of the time slot may be lost due to either saturation or severe quantization.
There are many existing AGC loop designs, such as the analog-monitored signal AGC loop and the digital-monitored loop, but in general the loops are designed to monitor the received signal at the ADC input, or output, and provide negative feedback to the analog variable receiver gain section in an attempt to maintain the monitored signal at a constant target level. In general, the measured characteristic of the monitored signal is the peak voltage, peak power, or mean power. If the measured characteristic of the monitored signal is higher than the target level, the analog gain of the receiver will be reduced according to the difference between the measured level and the target level; whereas, if the measured characteristic of the monitored signal is lower than the target level, the analog gain of the receiver will be increased accordingly.
However, this known approach has the following drawbacks: 1) In a packet-radio system such as a TDD-CDMA system, for a particular cell frequency, the power of a signal transmitted on a timeslot is, in general, a function of the number of codes transmitted. Thus, given the timeslot-segmented nature of the TDD-CDMA system, the power transmitted in each timeslot may vary considerably as the number of codes varies. The mobile station, although aware of its own timeslot/code allocations, is not usually aware of the allocations to the other users, and therefore cannot predict how much power will be received in a given timeslot. This therefore presents difficulties for AGC since it is the function of AGC to adjust the receiver analog gain in response to the received power such that a signal presented to the ADC is at an appropriate level. 2) For TDD systems, a further problem exists for AGC due to the TDD nature of the system. During the initial synchronization phase, the mobile station must search for a specific synchronization code transmitted by the network. At this point, the mobile station does not have any knowledge of the frame timing of the system. Due to the fact that uplink timeslots are transmitted on the same frequency as downlink timeslots (but are separated in time within the frame), without knowledge of the frame timing, the mobile station must configure itself to receive the signal on all timeslots in search for the synchronization code. The mobile station receiver is therefore subject to reception of uplink signals from nearby mobile stations on the same cell frequency. These uplink signals may be hundreds of times larger in power than the downlink signal that the mobile terminal is trying to detect. As a result, any AGC loop that tries to track the received signal power over the whole radio frame will try to accommodate a substantially large uplink signal and may consequently suppress the (relatively small) wanted downlink synchronization signal, thereby rendering such synchronization signal undetectable. This suppression of the wanted downlink signal is known as “blockage”.
Even if the signal is detectable, the synchronization correlation peak in timeslot 0 may be much smaller than the correlation noise peak occurring in the timeslot with highest power. This will result in a synchronization lock failure, or a false detection (which will also eventually lead to a synchronization failure).
Although this problem will not always exist, it is desirable to implement a receiver strategy that provides robustness under these adverse conditions, since an intermittent inability to acquire synchronization will obviously result in a high level of user dissatisfaction. Such conditions are likely to occur in any environment where there is a high possibility of users being in close proximity to each other.
Furthermore, for TD-LTE terminals, there are mainly three challenges in AGC design. First, it is required to provide a high SNR system, for example, 64QAM for high data rate. Consequently, there is a more strict restriction on the ADC quantization noise. Second, TD-LTE has the TDD characteristics that the uplink/downlink (UL/DL) allocation is configurable. Consequently, it is more difficult to estimate received signal power, especially in cases of lack of information in timing and UL/DL configuration. Third, due to the dynamic scheduling at base station, the signal power varies fast in the downlink where the DL signal power is large when much bandwidth is used for transmission while the DL signal power is small when the DL bandwidth is not used for transmission.
Therefore, there remains a need in the art for a quick and accurate method for adjusting the analog signal gain at the input of the ADC in order to maintain the magnitude of the input signal and to prevent any non-detection of the wanted downlink synchronization signal, in particular to a TD-LTE system.